Source of gold and ore-forming processes in the Zarshuran gold deposit, NW Iran: Insights from in situ elemental and sulfur isotopic compositions of pyrite, fluid inclusions, and O-H isotopes

The Zarshuran gold deposit (155 t Au, average grade: 2.63 g/t), NW Iran, provides a new paradigm for understanding the multicomponent ore-forming processes and metallogeny of gold during the evolution of hydrothermal fluids. It is characterized by auriferous quartz veins and gold coexisting with disseminated Fe-As-S sulfide minerals that are hosted in a sequence of Early Cambrian metasedimentary rocks. A complex paragenesis is defined by five sulfide stages: pre-ore stage pyrite (Py0), early ore-stage pyrite (Py1 and Py2), middle ore-stage pyrite (Py3a, Py3b, and Py3c), late ore-stage pyrite (Py4), and post ore-stage pyrite (Py5). Py0 with framboidal texture is characterized by relatively low concentrations of As, Au, Cu, and Sb, and has a broad range of negative delta 34S values from -28.2 to -3.7 %o, indicating microbial reduction of marine sulfate. Py1 and Py2 show porous texture with As-rich bright bands also having high Au (mean: 18.4 ppm). Similarly, narrow ranges in delta 34S from -3.6 to +3.2 %o likely suggest a deep-seated magmatic sulfur source. In the middle ore-stage, Py3a cryptocrystalline cores have a range of positive delta 34S (+5.4 to +26.7 %o) values and high concentration of As(-Au), whereas later well-crystallized outer rim Py3c is Co(-Ni) rich, has a much more restricted delta 34S range (-2.9 to +1.4 %o), indicating an evolution of fluid composition from As-rich to Co-rich. Most of the gold (mean: 20.2 ppm) and trace elements (Hg, Ag, Sb, and Tl) were deposited in Py4, whose delta 34S values (+5.4 to +26.7 %o) show that ore fluid sulfur in this stage was mainly sourced from a mixture of magmatic sulfur with minor input of reduced sedimentary rocks. Low gold and trace element content with slightly negative delta 34S values (-8.5 to -4.2 %o) of Py5 show that they were likely formed from a metal-poor oxidizing fluid.The mineralizing fluid system can be described as carbonic-aqueous with low to moderate salinity (3.2-15.1 wt% NaCl equiv.) and medium temperature of 285 to 317 degrees C (early ore-stage) and 255 to 290 degrees C (late ore-stage), which suggests that phase separation was responsible for gold precipitation during late ore-stage As-Hg-Sb sulfide veins. The delta 18Ofluid ranges from -7.8 to +4.2 %o, and the delta D values for fluid inclusions in mineral range from -105 to -65 %o, suggesting involvement of meteoric water during late- to post ore-stages. Our results indicate that the Zarshuran is a distal disseminated gold deposit formed during southward subduction of the Proto-Tethys oceanic lithosphere beneath the northern margin of Gondwanan terranes through the early Cambrian.